Lubricating composition applied to boron nitride grinding wheels



United States. Patent 3,528,789 LUBRICATING COMPOSITION APPLIED T0 BORONNITRIDE GRINDING WHEELS Wilfred F. Mathewson, '.lr., Franklin, Mich.,and Robert S. Owens, Latham, N.Y., assignors to General ElectricCompany, a corporation of New York No Drawing. Filed May 15, 1968, Ser.No. 729,395 Int. Cl. B24b 29/00 US. Cl. 51295 3 Claims ABSTRACT OF THEDISCLOSURE Greatly improved grinding is accomplished by effectingrelative motion between the workpiece and a cubic crystal boron nitridegrain abrasive tool in contact with the workpiece while simultaneouslysupplying to the working surface of the abrasive tool a solid drylubricant from the group consisting of hexagonal boron nitride,lubricants having the general lamellar crystal structure of cadmiumiodide, molybdenum disulfide, and mixtures thereof. In the preferredembodiment the solid dry lubricant is supplied by contacting the workingsurface of the abrasive tool with a rod or other shaped body containingthe solid dry lubricant such that the lubricant is rubbed onto theabrasive tool as the grinding proceeds. The preferred composition forthe shaped lubricant body is a mixture of molybdenum disulfide, anactive diluent such as sulphur, an adhesive material to provide bettertransfer of the lubricant to the working surface of the abrasive tool,and a resin binder.

This invention relates to a method and composition for grinding wherebystock is removed from a workpiece by abrading it with an abrasive toolsuch as a grinding wheel in which the abrasive is cubic boron nitridewhile simultaneously applying one or more of certain dry lubricants tothe working surface of the grinding wheel.

Reference is here made to US. Pat. 2,947,617 issued Aug. 2, 1960, toRobert H. Wentorf, Jr., and assigned to the assignee of the presentinvention. The Wentorf patent discloses and covers cubic crystalstructure boron nitride having a zinc blende cubic structure and havinga hardness approaching the hardness of diamond. Such cubic crystal boronnitride grain is frequently referred to as borazon.

Because of its extreme hardness, borazon is a superb abrasive grainmaterial for grinding wheels and similar abrasive tools. [It can bebonded and formed into grinding wheels and the like either by means of ametal bond, an organic resin bond or by a vitreous bond.

It is known that in any abrasive system, friction represents a majorsource of energy consumption. Several effects can result from thisphenomenon. Friction is symtomatic of sliding contact which, in the caseof grinding, occurs at the abrasive crystal-workpiece interface. Thisintimate contact, along with the heat generated by friction, can causeabrasive or adhesive wear and can result in a detrimental chemicalreaction between the abrasive and the workpiece. 'In addition, the heatof friction can cause premature bond failure, thermal stress failure ofthe abrasive and undue workpiece damage. Hence, it is well known toapply lubricant to the grinding wheelworkpiece interface during grindingoperations. Generally liquid lubricants such as emulsifiable oils areused which additonally serve as a cooling medium; however, it is alsoknown that solid lubricants, for example graphite, can be used. Forexample, US. Pat. 3,321,287 discloses the use of a graphite lubricantwith grinding wheels of various types and reports improvements in wheellife of about 25%.

What we have discovered is that in the case of a borazon grinding tool,if certain solid dry lubricants are supplied to the working surface ofthe abrasive article during the grinding operation, an increase in[grinding efficiency is accomplished of an order of magnitude far beyondwhat would be expected. For example, in grinding ferrous base metalswith borazon grinding wheels, we have accomplished increases in grindingperformance in excess of l000% by the use of a solid dry lubricant inaccordance with this invention. The solid dry lubricants which providethe outstandingly excellent results with borazon are hexagonal boronnitride or a material having the lamellar crystal structure of cadmiumiodide or molybdenum disulfide. Examples of the latter are molybdenumdisulfide, tungsten disulfide and lead iodide. We do not purport to beable to explain with accuracy why such lubricant provides the unusualresults with borazon as the grinding abrasive. However, it is believedto involve the fact that borazon, in an oxidizing atmosphere and in aheated condition, develops a thin oxide surface layer. The solid drylubricant either minimizes any adverse effect from this phenomenon orcoacts with it to provide the great increase in efliciency. Moreover,grinding employs relatively high velocities between abrasive grain andworkpiece and, in addition, is a multi-chip rather than a singlechipoperation. Hence, friction and other interactions between grain andworkpiece can cause unusually high grinding wheel Wear with borazonbecause of its relatively high reactivity. An important aspect of thepresent invention is therefore believed also to reside in positioningthe lubricant between the abrasive grain and the chip. It should benoted that graphite is not included within the scope of the invention,albeith graphite is perhaps the best known and most widely used drylubricant and, by its nature, would appear to minimize oxidation of theboron nitride to the oxide. Yet, the fact is that graphite does not giveresults even closely comparable to the above solid lubricants. The solidlubricants that are included within the scope of the invention are, inaddition to hexagonal boron nitride, those solid lubricants which havethe general lamellar crystal structure of cadmium iodide or molybdenumdisulfide. These crystal structures are described in more detail atpages 278, 280 and 397 of Structural Inorganic Chemistry, A. F. Wells,Clarendon Press, 2nd edition, 1950. Page 278 of this publicationcontains the following listing:

Cd I structure: CdIg, 'CaI MrgI Ph l- Mnl Fel C01 Ybl MgBr- FeBr C0BrCdBr- NiBr TiCl TiI The solid dry lubricant can be supplied to theworking surface of the grinding wheel or other abrasive tool during thegrinding operation in any of a number of ways. For example, the solidlubricant can be formulated into the grinding tool itself. This wouldappear to be the best mode of practicing the invention, particularlyfrom the standpoint of convenience; however, we have found that in factbest results are achieved by applying the solid lubricant to the workingsurface of the abrasive tool with a solid easily abradable and thereforerelatively soft body consisting of or containing the solid lubricant.\One advantage of this is that it enables the application of a fargreater amount of the solid dry lubricant to the working surface thancan be accomplished, as a practical matter, by including the solidlubricant in the abrasive tool itself. Hence, in the preferredembodiment of the invention the working surface of the [grinding tool,during its rotary or other motion in the grinding operation, is incontact with a rod of easily abradable material containing the solid drylubricant such that as the working surface of the abrasive tool engagesthe workpiece, it has a thin deposit of the solid dry lubricant thereon.This places the lubricant between the abrasive grain and the chip.'Ihere need be no other lubrication nor need cooling fluid be applied,as by the use of liquid lubricants, though such can be used if desired.

In the most preferred embodiment, the relatively soft, easily abradablebody in contact wih the moving working surface of the borazon abrasivetool consists essentially of a resin-bonded mixture of the solid drylubricant plus an active diluent such as sulphur plus a material whichfunctions as an adhesive to better assure transfer and adherence of thesolid lubricant onto the surface of the tool. The preferred organicresins for use as a bonding medium for the lubricant body are thethermosetting resins, especially epoxy and polyester resins. Thethermoplastic resins, such for example as the acrylics and the vinyls,can also be used though they do not provide as strong a bond as doesepoxy or polyester resin. Certain of the thermosetting resins, forexample the phenolics, have the disadvantage, as compared with epoxy orpolyester resin, of being somewhat brittle.

As the adhesive ingredient we prefer to use a silane, gammaaminopropyltriethoxysilane being aspecially excellent. Other materialswhich serve well as the adhesive are the higher alcohols such asoctadecanol and triethanolamine. In general, solid or liquid organiccompounds having amine or hydroxyl groups function as adhesives byimproving the adherence of the solid dry lubricant to the grindingwheel. While the adhesive ingredient is generally desirable, it is notessential.

The precise proportions of ingredients used in the formulation of thebody should be such as to supply to the surface of the abrasive tool athin deposit of the solid dry lubricant and yet with the body havingsufficient strength and hardness to assure against breakage or otherrapid degeneration of the body during the grinding operation. We havefound it desirable to use from about 30 to 60% by volume organic bondingresin and at least about 30% by volume solid dry lubricant. Where anadhesive is included, only about 2 to 6% by volume need be used. Thefollowing composition, expressed in percent by volume, has been found tobe outstandingly excellent: Molybdenum disulfide about 32%, sulphurabout 22%, gamma aminopropyltriethoxysilane about 4%, epoxy resin about42%.

After thorough mixing, this composition is shaped into a cylindrical rodof say about .1 inch diameter and the epoxy resin then cured over aperiod of about 12 hours at room temperature with a 3-hour post cure at90 C. in the practice of the invention, such a rod is consumed at therate of about inches per hour at normal grinding speeds.

Specific examples of lubricant compositions made in accordance with theinvention are as follows:

1) Molybdenum disulfide 32.2%, sulphur 21.6%, aminopropyltriethoxysilane3.8%, epoxy resin 42.4%.

(2) Same as (1) except lead iodide substituted for the molybdenumdisulfide.

(3) Hexagonal crystal powdered boron nitride 38.6%,aminopropyltriethoxysilane 5.4%, epoxy resin 56%.

Grinding efficiency can be determined and expressed in terms of thegrinding ratio, i.e., the ratio between the rate of stock removal andthe rate of grinding wheel wear. The higher the ratio, the greater theefiiciency. With the present invention, increases of as high as l000% ingrinding ratio can be accomplished. To illustrate, in grinding M2 highspeed steel with an unlubricated resin-bonded borazon grain grindingwheel, the grinding ratio was 33. Using an identical grinding wheel andwith all other conditions being identical except that the grinding wheelwas lubricated by a rod formed of composition 1 above, the grindng ratiowas 400'.

Whereas it is much preferred to apply the solid dry lubricant to theworking surface of the abrasive tool from a solid, easily abradablebody, the lubricant can, if desired, be formulated into the abradingtool itself. Where this is done, it is desirable that the solid drylubricant be formulated into the abrading tool in an amount of fromabout 5% to 25% by volume of the total constituents of the abrading toolcomposition. The following specific example will serve to illustrate,the percentages being by volume:

Percent Borazon (60 to mesh) 10 Molybdenum disulfide l5 Theseingredients were homogeneously mixed, pressed into a grinding wheel andcured. The grinding ratio was 62 as compared with a grinding ratio ofabout 30 without the inclusion of the molybdenum disulfide.

We claim:

1. A lubricating composition for application to cubic crystal boronnitride grain grinding wheels to provide lubrication at the surface ofsaid wheels during grinding, said composition comprising a mixture offrom about 30% to 60% by volume thennosetting organic bonding resin andat least about 30% by volume solid dry lubricant selected from the groupconsisting of hexagonal born nitride, molybdenum disulfide, or aninorganic metal halide having the lamellar crystal structure of cadmiumiodide or molybdenum disulfide, and mixtures thereof, the balance ofsaid mixture being composed of essentially non-abrasive materials.

2. A lubricating composition as claimed in claim 1 wherein the solid drylubricant is selected from the group consisting of molybdenum disulfide,lead iodide, and hexagonal crystal powered boron nitride.

3. A lubricating composition as claimed in claim 1 containing about 32%by volume molybdenum disulfide, 22% by volume sulfur, 4% by volume gammaaminopropyltriethoxysilane, and 42% by volume epoxy resin.

References Cited UNITED STATES PATENTS 2,240,302 4/ 1941 I ones 512952,881,065 4/19'5'9 Renter 5 l--298 2,947,617 8/1960 Wentorf 512983,081,161 3 1963 Cantrell 5 1298 3,383,191 5/ 1968 Thomas 151-2983,385,684 5/1968 Voter 51-298 "582,736 5/1897 Haas 51-295 2,904,4199/1959 Couch et al. 51-298 3,27 6,170 10/ 1966 Crowe 5 1-295 DONALD J.ARNOLD, Primary Examiner U.S. Cl. X.R. 51298, 307

